In view of the recent significant development of the nuclear industry, a variety of problems have emerged with respect to potential hazard and exposing workers to radioactive materials and radiation in nuclear plants. During the periodic maintenance and repair work carried out in nuclear power stations, it is absolutely necessary not only to protect workers from intense radiations such as gamma (.gamma.) rays, but also from exposure to even the slightest amount of neutrons, which can radiate from the nuclear reactor in the event of an emergency.
The nuclear industry has thus urgently desired to have available neutron shielding materials of high flexibility and desirable operative properties for incorporation into garments, so that workers in nuclear plants and industrial sites can wear protective garments made of neutron-shielding material.
The importance of this invention has recently increased in view of the various experiments relating to the application of neutron radiation to medical treatment, such as neutron capture therapy, where a certain amount of neutrons is irradiated to a cerebral tumor so that only the tumor portion is effected and can thus be removed. During this surgical procedure, it is essential to protect the rest of the patient's body from the neutron radiation itself as well as to the secondary radiation produced when neutrons strike their target. In this case, it is an urgent need in the art to fabricate neutron-shielding materials in the shape of fabrics such as bandages, gauzes and blankets. This demand can potentially be satisfied only by the use of fiber materials having the necessary neutron shielding properties.
Further, in the event that neutron weapons are used in warfare, although the neutron-shielding fabrics and fibers of this invention will not shield against any of the fast neutrons emitted therefrom, if people wore protective robes of the neutron-shielding fibers of this invention and were situated in shelters that provide the effect of reducing the energy of fast neutrons so as to slow them to thermal neutrons, peoples' lives could be saved by the shielding effect provided by the fabrics of this invention. The neutron-shielding material in accordance with this invention potentially have any possible utility relating to the protection of humans, animals and inanimate objects from thermal neutron radiation.
Conventional neutron-shielding materials are in the form of boards composed of cadmium and boron compound. However, such neutron-shielding boards are physically rigid and have no flexibility at all; furthermore, since cadmium yields high secondary gamma rays upon absorbing neutrons, it is not suitable for use in shields for protecting the human body against neutron radiation.
Japanese laid-open patent applications Nos. 52-127597 and 52-131097 disclose neutron shielding materials formed in sheets of various kinds of plastics with boron and/or lithium compounds therein, which are disclosed to yield low levels of secondary gamma radiation in the occasion of neutron absorption. However, these products are not flexible enough for use in any of the protective clothing and the like contemplated by this invention. Another Japanese laid-open patent application, No. 53-21398, discloses a method of manufacturing neutron shielding fibers which consist either of ion exchange fibers which have absorbed therein ionized compounds of boron and lithium or of staple-like fibers containing therein boron and/or lithium compounds. In the case of the ion exchange fibers, the finished products cannot satisfactorily achieve the intended shielding properties to either the incomplete absorption and fixation of the neutron-shielding ionized compounds into the ion exchange fibers, or to the possible releasing of the once-fixed ionized compounds from the fibers during the washing and rinsing the fabrics embodying these fibers.
In the case of staple fibers, the finished products thus obtained by means of jet-spinning of this mixture of neutron-shielding inorganic compounds and fiber-forming polymers can physically retain the fibrous form. However, these products are not suitable for processing with any of the yarn-spinning and knitting or texturizing machines due to insufficient tensile strength, elongation, and textured styles. In addition, the finished products thus obtained usually have those neutron-shielding compounds exposed on the surface, and thus they can easily be stripped off from the surface, thus inevitably resulting in degraded shielding properties.
We have carried out extensive experiments with fibers composed of certain fiber-forming polymers, each having certain grading and neutron-shielding properties. As a result, we found that a variety of critical problems potentially existed. For example, certain neutron-shielding compounds deposited and existed on the surface and the adjacent portions of the fibers were found to be stripped off in processing, thus causing damage on the surfaces of guide rollers and other rollers due either to staining or to friction of the fibers against them. Consequently, not only can the production of the neutron-shielding fibers of stable quality not be achieved, but the finished fibers will have poor mechanical properties. In addition, neutron-shielding garments made of said compound fibers exhibit eventual stripping off of the deposited neutron-shielding compounds during and after wash and from friction of the fabric against objects.
We also found that, when these prior finished products composed of neutron-shielding fibers were exposed to neutron rays, certain secondary radioactive materials were generated by the nuclear reaction. For example, when lithium (Li) compounds were applied to the neutron-shielding compound, the lithium compounds exposed to the thermal neutron rays irradiated onto the fiber surface then generated a certain amount of tritium which then started to diffuse in the atmosphere.